1. Field of theInvention
The present invention relates to an optical waveguide modulator, and more particularly, to the provision of improved thermal and temporal bias stability in optical waveguide devices.
2. Discussion of the Related Art
Mach Zehnder interferometers (MZI's) used as optical modulators are of great interest for high data rate fiber optical communications systems. A great deal of research has been carried out to develop this type of device since its introduction in the mid-70's. The practicality of Ti-diffused LiNbO3 waveguide systems has allowed wide introduction of these devices in current optical communication systems.
FIG. 1 illustrates a plan view of a related art Z-cut lithium niobate Mach-Zehnder interferometer used for modulation of an optical signal. An optical waveguide path 4 is formed inside a surface of a lithium niobate (LiNbO3) substrate 1 that splits into a first path 4a and a second path 4b and then recombines back into a single path 4′. The optical waveguide paths 4a and 4b may be formed by diffusion of a metal, for example titanium, or with other dopants that will form an optical path in the lithium niobate substrate 1. An electric field is applied to the first optical waveguide path 4a and the second optical waveguide path 4b via electrodes 2a and 3, respectively, that are positioned over the first and second optical waveguide paths. Specifically, the electrode 2a over the first optical waveguide path 4a is a ground electrode and the electrode 3 over the second optical waveguide path 4b is an input electrode. In addition, another ground electrode 2b is positioned on the substrate so that ground electrodes 2a and 2b are on each side of the input electrode 3 for further control of the electric fields applied to the first and second optical waveguide paths 4a and 4b. The electrodes 2a, 2b and 3 are separated from the substrate 1 by a buffer layer 5. The application of the electric field changes the refractive index of an optical waveguide path in proportion to the amount of electric field applied. By controlling the amount of electric field applied via the electrodes 2a, 2b and 3, an optical signal passing through the optical waveguide paths can be modulated.
FIG. 2 is cross-sectional view of the related art Z-cut lithium niobate Mach-Zehnder interferometer along line B-B′ in FIG. 1. The buffer layer 5 is comprised of a transparent dielectric film and is positioned between the electro-optical crystal substrate 1 and the electrodes 2a, 2b and 3. The buffer layer 5 prevents optical absorption of the optical mode by the metal electrodes 2a and 3. However, the buffer layer 5 allows electric fields that emanate from the electrode 3 to affect a refractive index change in either or both the first optical waveguide path 4a or the second optical waveguide path 4b. Typically, silicon dioxide (SiO2) is used as the buffer layer due to its optical transparency at 1.55 microns and its low dielectric constant.
FIG. 2 also illustrates that electro-optical crystal substrate 1 of the related art Z-cut lithium niobate Mach-Zehnder interferometer is formed so that a Y axis of the crystal orientation extends in a longitudinal direction of the lithium niobate substrate 1 along the waveguide paths 4a and 4b. The Z axis of the crystal orientation extends in the direction of the thickness of the electro-optical crystal substrate 1 such that the top and bottom surfaces of the lithium niobate substrate 1 are respectively −Z and +Z faces in terms of the crystal lattice structure of the substrate. The optical waveguide paths are commonly denoted as being within the −Z face of the lithium niobate substrate.
One of the practical difficulties in the early introduction of Z-cut LiNbO3 devices was the pyroelectric sensitivity of LiNbO3, which resulted in the development of large internal fields within the devices when subjected to temperature changes or gradients across the device. This is because a change in temperature causes a change in the spontaneous polarization due to the ferroelectric properties of LiNbO3. As illustrated in FIG. 2, this results in an imbalance of charge between the Z faces of the electro-optical crystal substrate 1, so that an electric field is generated in the Z direction perpendicularly along the waveguide paths 4a and 4b of the device. Due to the very high resistivity of LiNbO3, these charges take a long time to travel through the electro-optical crystal substrate 1 and neutralize themselves. This imbalance of charge impedes or lessens the effect of the electrical fields from the electrodes 2a, 2b and 3 on the waveguide paths 4a or 4b, thus decreasing the effectiveness or control in modulating optical signals. Early modulators were highly susceptible to thermal changes and strict environmental controls were necessary for thermal stabilization of the devices.
An early approach to maintain or prevent loss of modulation control due to thermal effects was to bleed off or counteract the imbalance of charge between the Z faces of a LiNbO3 substrate. C. H. Bulmer et al. (one of the authors is an inventor in this application), “Pyroelectric Effects in LiNbO3 Channel Waveguide Devices,” Applied Physics Letters 48, p. 1036, 1986 disclosed that metallizing the Z faces, and electrically connecting them with a high conductivity path to allow the unbalanced charge to neutralize rapidly, resulted in improved thermal stability of an X-cut device. Nonetheless, in Z-cut devices, this approach is difficult since the waveguide paths are on the Z face, and a metalized layer on this face would short out the electrodes of the device, making the device ineffective or inoperable.
Instead of a metallization layer, P. Skeath et al. (one of the authors is an inventor in this application), “Novel Electrostatic Mechanism in the Thermal Stability of Z-Cut LiNbO3 Interferometers,” Applied Physics Letters 49, p 1221, 1986 and I. Sawaki et al., Conference on Lasers and Electro-Optics, MF2, PP. 46-47, San Francisco, 1986 suggested a semiconducting or semi-insulating layer on the Z face under the electrodes of a Z-cut device. The semiconducting or semi-insulating layer would transfer the unbalanced charge between the Z faces of the LiNbO3 substrate but not short out the electrodes. Although X-cut devices are commonly treated by providing metal layers or other conductive layers on the Z faces and interconnecting the conductive layers, research continues as to what semiconductor or semi-insulating layer can be best or appropriately specified for use with Z-cut devices.
Approaches attempted in the past have included Indium Tin Oxide (ITO), Silicon (Si), and Silicon Titanium Nitride (SixTiyNz) layers, which are applied in place of or above the usual SiO2 buffer layer on a Z-cut optical waveguide device. Minford et al., “Apparatus and Method for Dissipating Charge from Lithium Niobate Devices, U.S. Pat. No. 5,949,944, Sep. 7, 1999, which is hereby incorporated by reference, proposes a silicon titanium nitride layer that has the advantage of adjustable resistivity by adjustment of the silicon/titanium ratio. However, control of the resistivity is unsatisfactory due to oxygen contamination in the silicon titanium nitride buffer layer, which results from residual background gases in the deposition system. This results in unacceptable run-to-run variation in the resistivity of a silicon titanium nitride buffer layer. Furthermore, the deposition system for a silicon titanium nitride buffer layer includes a sputtering process that requires a variety of targets with varying compositions to vary the composition of the buffer layer over a desired range and thus, is not a practical process with suitable control of the resistivity.
The effect of the electric field and the consistency of the effect of the electric field over time (i.e. temporal stability) applied to the waveguide paths are greatly affected by characteristics of the buffer layer. The amount of electric field from the electrodes that is affected by charge variations within the buffer layer or by the charge imbalance in the lithium niobate substrate is referred to as the bias drift of a device. Temporal stability of Z-cut Ti diffused LiNbO3 devices has been discussed in Seino et al., “Optical Waveguide Device,” U.S. Pat. No. 5,404,412, Apr. 4, 1995, which is hereby incorporated by reference. Specifically, Seino et al. shows that repeated or constantly applied voltages across a buffer layer in a Z-cut device will ultimately result in a buffer layer developing charge screening processes that significantly reduces the electric field across the waveguide paths. Seino et al. further shows that by adding titanium and indium oxides to an SiO2 buffer layer, the resulting bias drift was reduced (delayed in time).